It is often useful and necessary for medical or research reasons to obtain images from within a subject. An endoscope or some other medical probe has the ability to provide images from inside the subject. The subject may be a human patient.
The endoscope may include a fiber probe. One or more optical fibers may be utilized to navigate, deliver light to, and receive light from a sample (such as body, organs, tissues and any other materials). The fiber probe may also include a lens and a mirror assembled at a distal end of the fiber to focus light, collect light, and guide the light.
Endoscopes have been developed so as to gain access to internal organs with minimal evasion of the subject. For example in cardiology, OCT (optical coherence tomography) has been developed to see depth resolved images of blood vessels via a catheter. The catheter, which consists of a sheath, a coil, and a fiber optical probe, is navigated to an area of interest such as a coronary artery. As the beam of light from a fiber optical probe is rotated across a surface of interest, cross-sectional images of the vessels are obtained. In order to acquire three-dimensional data, the optical probe is simultaneously translated longitudinally during the rotation so that images based on a helical scanning pattern are obtained. This helical translation may be performed by pulling the tip of a fiber optic probe back towards a proximal end, while it is simultaneously being rotated, and therefore is referred to as a pullback.
The catheter includes a clear sheath in which light from the fiber optic probe passes. The smaller the diameter of the probe, the stronger the optical power that the clear sheath adds along one axis of the optical system, and the greater the astigmatism of the optical system.
U.S. Pat. Nos. RE43,875 and RE45,512 disclose methods in which the refractive index of a sheath substantially matches the refractive index of the medium outside the sheath and a fluid inside the sheath. However, a problem with this method is the lack of accountability for the viscosity and turbulence due to the fluid inside the sheath, while the probe is being pulled back. U.S. Pat. Nos. RE43,875 and RE45,512 also disclosed adding a prolate spheroidal ball to the end of the endoscope to compensate for the astigmatism of the sheath. U.S. Pat. No. 8,582,934 discloses compensating for the astigmatism of the sheath with a curved reflective cap with two different focal points. However, these systems can be difficult to align, manufacture, and maintain.